The present invention relates to an anti-lock braking control method and apparatus for preventing locking of automotive wheels of a running vehicle during braking.
Generally, in an anti-lock control apparatus for automotive vehicles, to maintain steering ability and running stability during braking, a control unit including a microcomputer controls brake hydraulic pressure. This brake pressure control also reduces braking distance.
During operation of the anti-lock control apparatus, a control mode for brake hydraulic pressure is determined based on an electric signal indicative of a wheel speed. This wheel speed is detected by a wheel speed sensor. According to the control mode, a hold valve, which is a normally opened solenoid valve, and a decay valve which is a normally closed solenoid valve, are selectively opened or closed to increase, hold constant and decrease the brake hydraulic pressure.
In such anti-lock control devices, as disclosed in the U.S. Pat. No 4,984,164, a reference speed for deciding when to decrease the brake hydraulic pressure is set or determined in accordance with a wheel speed (herein after referred as system speed Vs) to be controlled in each brake control system or channel. There is also set an estimated vehicle speed Vv having a predetermined follow-up limit of acceleration/deceleration with respect to the highest wheel speed among four wheel speeds. Further, there is set a threshold speed which tracks the estimated vehicle speed Vv with a predetermined speed difference therefrom and which is below the estimated vehicle speed Vv.
Based on the comparison of the system speed Vs with the reference speed or the threshold speed and also on the detected high peak point and low peak point of the system speed Vs, a pressure decrease status, a pressure hold status and a pressure increase status are set. Then, a plurality of hydraulic pressure valves (solenoid valves) are turned on and off in accordance with a predetermined control mode set for each of these statuses, thereby effecting the pressure decrease, pressure hold and pressure increase.
However, since the brake hydraulic pressure for each wheel, which is subjected to the anti-lock control through the corresponding brake control channel, is subjected to regulation such as increase, decrease and holding, without regard to the state of the regulation of the brake hydraulic pressure for another wheel, which is subjected to the anti-lock control through another corresponding brake control channel, all the brake control channels often act simultaneously with each other to start the increase in the brake hydraulic pressure for the four wheels of the vehicle at the same time. For that reason, when the speeds of the four wheels have dropped simultaneously with each other and because the estimated vehicle speed Vv is set on the basis of the highest one of the wheel speeds and each of the wheel speeds is compared with the estimated vehicle speed to perform the anti-lock control, the estimated vehicle speed becomes so different from the actual speed of the vehicle that the anti-lock control cannot be accurately performed.
Besides, when the four wheel speeds have dropped simultaneously with each other, moment of yawing, pitching or the like is likely to act to the vehicle to make the vehicle unstable.